grep: remove redundant REG_NEWLINE when compiling fixed regex

Remove the redundant REG_NEWLINE regcomp() flag from the code that
compiles a fixed-string regular-expression.

The REG_NEWLINE causes metacharacters such as "." to match a newline,
since the basic_regex_quote_buf() function being called here escapes
all metacharacters using REG_NEWLINE is confusing and redundant.

The use of this flag was introduced as an unintended emergent property
of 793dc676e0 ("grep/icase: avoid kwsset when -F is specified",
2016-06-25).

That change amended the existing regflags, which were initialized to
REG_NEWLINE in init_grep_defaults() assuming a subsequent non-fixed
regcomp().

Manual testing reveals that this was always redundant, since no flags
of any use were inherited from opt->regflags even back
then. 793dc676e0 passes all tests with this on top:

    diff --git a/grep.c b/grep.c
    index 627ae3e3e8..89e84ed7fd 100644
    --- a/grep.c
    +++ b/grep.c
    @@ -407,3 +407,3 @@ static void compile_fixed_regexp(struct grep_pat *p, struct grep_opt *opt)
            basic_regex_quote_buf(&sb, p->pattern);
    -       regflags = opt->regflags & ~REG_EXTENDED;
    +       regflags = 0;
            if (opt->ignore_case)

Since this isn't used for anything and never was, remove it to reduce
confusion when reading this code.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -593,7 +593,7 @@ static void compile_fixed_regexp(struct grep_pat *p, struct grep_opt *opt)
 {
 	struct strbuf sb = STRBUF_INIT;
 	int err;
-	int regflags = REG_NEWLINE;
+	int regflags = 0;
 
 	basic_regex_quote_buf(&sb, p->pattern);
 	if (opt->ignore_case)